Lab 2: Diodes

Indy Hendrickx21/08/2026About 15 min

Lab 2: Diodes

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1. Assignment description

In lab 1 you worked with a voltage that stayed put. In this lab the voltage moves, and a moving voltage needs two new instruments: the function generator to make it, and the oscilloscope to see it. Once you can drive and read those, you build the circuit that sits inside every power supply in the building: a bridge rectifier.

The order is deliberate. First the instruments, then one diode, then four.

What we expect:

Every measurement table filled in, with units
Every resistor you use calculated first, not guessed
Photos of your circuits, and screenshots or photos of the scope screen where indicated
A short reflection of approximately half a page in Chapter 3

Planning

When	Steps	What you do
Theory lesson (last 1,5 h)	2.1 to 2.4	Function generator, oscilloscope, reading a signal off the screen, testing diodes and LEDs
Lab session (3 h)	2.5 to 2.9	LED with a calculated resistor, half wave rectifier, bridge rectifier, smoothing capacitor, LED on the output
If you finish early	2.10 to 2.12	RGB LED, ripple against load, other waveforms. Not graded

What do we need for the whole lab?

From your own kit:

1x breadboard and jumper wires
Resistors: 100 Ω, 220 Ω, 1 kΩ, 4,7 kΩ, 10 kΩ
2x red LED, 1x RGB LED
1x 1 µF capacitor

From the lab room:

1x bench power supply, 1x multimeter with test leads
1x function generator with a BNC test lead (crocodile clips)
1x oscilloscope with a ×10 probe
4x diode 1N4148 or 1N4007 (you get these from the teacher, they are not in your kit)
1x electrolytic capacitor 100 µF, 25 V or higher

Safety rules for this lab

A scope ground clip may only touch the ground of the function generator. Step 2.7 explains why. This is the one rule in this lab that can damage equipment.
The electrolytic capacitor has a polarity. The stripe marks the − leg. Wrong way round means heat, smell and a bang.
An LED never goes into a circuit without a series resistor, not even for a quick test.
On the bench supply, remember from lab 1: there is no output button, so switching off means turning the voltage down to 0 V and pulling the wire out of the + rail.
Everything in this lab runs on a few volts from an instrument. Mains voltage is never part of the exercise.

2. Lab

2.1 The function generator

A bench supply makes one fixed voltage. A function generator makes a voltage that changes shape over time: a sine, a square or a triangle, at a frequency and an amplitude you choose. It is your AC source for the upcoming labs.

Front panel of the function generator

Nr	Control	What it does
1	Display	Shows the frequency it is putting out. It does not show the amplitude
2	Waveform	Sine, square or triangle
3	Range	Multiplier for the frequency dial. Pick the range first, then turn the dial
4	FREQUENCY dial	Sets the frequency inside the chosen range, coarse and fine
5	AMPLITUDE	Sets how big the signal is. No readout, so you set it with the scope
6	DC OFFSET	Shifts the whole signal up or down. Stays off in this lab
7	ATT −20 dB	Attenuator, makes the output ten times smaller. Stays out in this lab
8	MAIN OUT	The BNC output you use. The outer ring of the BNC is ground
9	TTL OUT	A fixed 5 V square wave for digital circuits. Not used today

Step 1: Find the controls on your own unit

Your generator may put these in a different order or give them slightly different names. Before you switch anything on, find each of the nine controls and write down where it is.

Nr	Control	Where is it on your unit?
1	Frequency display
2	Waveform selection
3	Range
4	Frequency dial
5	Amplitude
6	DC offset
7	Attenuator
8	Main output

Step 2: Make your first signal

Switch the generator on.
Choose sine.
Choose the range that contains 1 kHz and turn the dial until the display reads 1,000 kHz as closely as you can.
Turn the AMPLITUDE knob to about the middle.
Make sure DC OFFSET is off and the attenuator is out.

Look at the display. Which of the two settings that matter, frequency and amplitude, can you actually read on this instrument?

💡 Look again at what is printed next to each knob.

2.2 The oscilloscope

The multimeter gives you one number per measurement. The oscilloscope draws the shape of a voltage against time, which is the only way to see what a diode is really doing to a signal.

Front panel of the oscilloscope

Nr	Control	What it does
1	Screen	A grid of divisions: 10 wide, 8 high. Everything is read in divisions
2	AUTOSET	The scope tries to find a stable picture by itself. Your rescue button
3	CURSOR	Two lines you move over the trace to measure a distance yourself
4	MATH	Calculates with the channels, for example CH1 − CH2. You need this in step 2.7
5	VOLTS/DIV	How many volts one division is worth, per channel. The vertical zoom
6	CH1 / CH2 MENU	Switches a channel on or off, sets coupling and probe factor
7	SEC/DIV	How much time one division is worth. The horizontal zoom
8	TRIGGER	Decides where the scope starts drawing, so a repeating signal stands still
9	CH1 and CH2 inputs	BNC connectors for the probes
10	PROBE COMP	A built-in test signal: a 5 V square wave at 1 kHz

Step 1: The probe

A scope probe is not a piece of wire. It contains a resistor network that divides the signal by ten, which is why it is called a ×10 probe. The scope has to be told about that, otherwise every voltage you read is ten times too small.

Plug the probe into CH1.
Check the switch on the probe body: set it to 10X.
Press CH1 MENU and set the probe factor in the menu to 10X as well.

Step 2: Compensate the probe

Clip the probe tip onto the PROBE COMP terminal and the ground clip onto the ground tab next to it. Press AUTOSET. You should see a square wave.

A square wave that is under compensated, correct, and over compensated

If the corners are not square, turn the small trimmer screw on the probe body with a small screwdriver until the top of the square wave is flat.

Take a picture of your screen after compensating.

Step 3: Read the screen without the automatic measurements

Everything on a scope screen is counted in divisions. The two knobs tell you what one division is worth.

Reading a signal off the graticule

In the picture above:

The wave is 6 divisions high and CH1 is set to 2 V/div, so U peak-to-peak = 6 × 2 = 12 V
One period is 4 divisions wide and the scope is set to 1 ms/div, so T = 4 × 1 = 4 ms and f = 1 / 0,004 = 250 Hz

Now do the same with the PROBE COMP signal on your own screen.

Reading	Your answer
VOLTS/DIV setting
Height of the square wave in divisions
U peak-to-peak (calculated)
SEC/DIV setting
One period in divisions
Period T (calculated)
Frequency f (calculated)

The PROBE COMP output is specified as 5 V at 1 kHz. Do your calculated values match? If not, check the probe factor first.

2.3 The generator and the scope together

Now connect the two instruments and check whether the generator does what it claims.

Connect: the BNC lead from MAIN OUT of the generator to the breadboard, black clip to the − rail, red clip to the + rail. Put the CH1 probe tip on the + rail and its ground clip on the − rail.

Step 1: Set an exact amplitude

The generator cannot show you its amplitude, so use the scope for it. Set a sine of 1 kHz and turn the AMPLITUDE knob until the scope shows exactly 6 V peak-to-peak.

Use MEASURE to help: press MEASURE, choose CH1 and the measurement Pk-Pk, and a second one for Freq.

Quantity	Read from the divisions	Read with MEASURE
U peak-to-peak
U peak
Period T
Frequency f

Step 2: Three frequencies

Keep the amplitude at 6 Vpp and set the generator to each frequency below. Each time, adjust SEC/DIV so that you see two or three periods, and read the period off the screen.

Generator display	SEC/DIV you used	Period from the screen	Frequency you calculate
100 Hz
1 kHz
10 kHz

Step 3: The same signal on the multimeter

Set the generator to 100 Hz and 6 Vpp. Measure the same signal with the multimeter in V AC mode (probes on the same two rails).

Measurement	Value
U peak-to-peak on the scope
U peak (calculated)
U RMS you expect (U peak × 0,707)
U AC on the multimeter

The scope and the multimeter show different numbers for the same signal. Is one of them wrong? Explain what each instrument is telling you.

💡 Look up in the theory which value a multimeter shows in AC mode.

Your meter has a frequency limit

A cheap multimeter is built to measure the mains at 50 Hz. Feed it 10 kHz and the reading quietly drops, even though the signal has not changed. Check the manual of your meter for the frequency range of its AC ranges before you trust an AC reading.

2.4 Getting to know the diode

Before you build anything, get to know the component itself.

What do we need? 2x diode, 1x red LED, 1x green or yellow LED, 1x blue or white LED (or the RGB LED from your kit), 1x 1 kΩ resistor.

Step 1: Which side is which?

Look at the diode. One end has a printed ring around the body. That ring marks the cathode, the side the current comes out of. On an LED the marking is different: the short leg is the cathode, and the plastic rim is flattened on that side.

Step 2: The diode test on the multimeter

Your multimeter has a diode symbol on the rotary switch. In that position the meter pushes a small current through the component and shows you the voltage it needs.

Testing a diode in both directions

Test each component in both directions and write down what the display shows. "OL" means no current gets through.

Component	Red probe on the plain side	Red probe on the ring / short leg
Diode 1
Diode 2
Red LED
Green or yellow LED
Blue or white LED
1 kΩ resistor

Which components lit up faintly while you were testing them, and which one refused to show a value at all? Use the forward voltage table from the theory to explain why.

💡 The meter only has a few volts available for its test.

Why does the resistor behave differently from all the others in this test?

2.5 An LED with a resistor you calculate yourself

This is the calculation you will repeat for the rest of your career, so do it properly once.

LED with a series resistor on the bench supply

What do we need? 1x red LED, resistors from your kit, bench supply, multimeter.

Step 1: Calculate

You want 10 mA through a red LED on a 5 V supply. Use the forward voltage you measured yourself in step 2.4, or 2,0 V if your measurement looked odd.

R = \frac{U_{\text{supply}} - U_{\text{LED}}}{I_{\text{LED}}}

Then pick the closest value above your result from your kit (100 Ω, 220 Ω, 1 kΩ, 4,7 kΩ, 10 kΩ) and calculate what current that resistor will really give.

Step 2: Build and measure

Voltage to 0 V. Build the circuit: + rail, resistor, LED (long leg towards the resistor), − rail.
Set the supply to 5 V with the current limit at 100 mA.
Measure and fill in the table.

Quantity	Predicted	Measured
U over the resistor
U over the LED
U over R + U over LED
I through the circuit (calculated from U over R)

Take a picture of the working circuit.

Step 3: Change the resistor

Swap your resistor for a 1 kΩ and repeat.

Quantity	Calculated	Measured
U over the LED
I through the circuit
Brightness (dim, normal, bright)

The LED voltage barely moved while the current changed a lot. Which component in this circuit decides the current, and which one decides the voltage?

Compare the LED voltage you measured here with the value the multimeter showed in diode mode in step 2.4. Are they the same? Why not?

💡 How much current does a multimeter push through in diode mode, and how much is flowing now?

2.6 Half wave rectifier

One diode, one resistor, and your first rectifier.

Half wave rectifier with both scope probes

What do we need? 1x diode, 1x 1 kΩ resistor, function generator, scope with two probes.

Step 1: Build it

Generator to the breadboard: black clip to the − rail, red clip to a free column. Call that column the input.
Diode from the input column to a second column, with the ring pointing away from the input.
Resistor from that second column to the − rail.
CH1 probe tip on the input column, CH2 probe tip on the second column.
Both ground clips on the − rail, which is also the generator ground.
Generator: sine, 100 Hz, amplitude to maximum.

Press AUTOSET, then set both channels to the same VOLTS/DIV so you can compare them fairly.

Step 2: Look and measure

Measurement	Value
U peak of the input (CH1)
U peak of the output (CH2)
Difference between the two
What CH2 does during the negative half of CH1

Take a picture of the screen with both traces visible.

Step 3: What does the multimeter make of it?

Measure the output with the multimeter in V DC mode, across the 1 kΩ resistor.

Measurement	Value
U DC on the multimeter
0,318 × U peak of the output (calculated)

Step 4: Turn the amplitude down

Reduce the generator amplitude until the input is only about 2 V peak-to-peak, and watch CH2.

What happens to the output, and why does the diode suddenly ruin so much more of the signal?

Turn the amplitude back to maximum before you continue.

2.7 Bridge rectifier without a capacitor

Four diodes, and both halves of the wave reach the load. This is the core circuit of this lab.

First read this, before you clip anything on

The ground clip of a scope probe is connected to the metal case of the scope, which is connected to the earth pin of its power plug. The ground of the function generator is earthed in exactly the same way. All those grounds are one and the same point, even though they are separate crocodile clips.

In the circuits so far that was harmless, because the generator ground and the load both sat on the − rail. In a bridge rectifier neither output rail is at ground, and clipping a probe ground onto one of them short circuits a diode:

Wrong: a scope ground clip on the minus rail of a bridge

Rule for the rest of this lab: both ground clips go to the generator ground, and nowhere else. To see the voltage across the load you use two probes and let the scope subtract them.

Step 1: Build the bridge

Bridge rectifier with the probes on the two rails

Use four columns on your breadboard: node A, node B, the + rail and the − rail. Then place the four diodes exactly as in this table. Watch the ring on every single one.

Component	From	To	Ring (cathode) side
D1	node A	+ rail	+ rail
D2	node B	+ rail	+ rail
D3	− rail	node A	node A
D4	− rail	node B	node B
Generator black clip	node A
Generator red clip	node B
R load 1 kΩ	+ rail	− rail

All four rings point towards the + rail, or away from the − rail. If one of them points the other way, the bridge does not work.

Step 2: Measure with MATH

CH1 probe tip on the + rail, CH2 probe tip on the − rail.
Both ground clips on node A (the generator ground).
Set CH1 and CH2 to the same VOLTS/DIV. The subtraction is only meaningful if both channels use the same scale.
Press MATH and select CH1 − CH2.

The MATH trace is the voltage across your load.

Measurement	Value
U peak of the generator (CH1 to CH2 before the bridge, or from step 2.6)
U peak of the MATH trace
Difference between the two
Number of bumps you count in one period of the input
U DC across the load on the multimeter
0,637 × U peak of the output (calculated)

Take a picture of the screen with the MATH trace.

In the half wave circuit you lost about 0,7 V. Here you lose about twice as much. Explain where the second 0,7 V goes.

Compare the DC voltage on the multimeter with the one you measured in step 2.6. Which rectifier delivers more, and roughly by how much?

2.8 The smoothing capacitor

The output is DC, but it is still a row of bumps. Time to flatten it.

What do we need? the circuit from 2.7, plus the 100 µF electrolytic capacitor and the 1 µF capacitor from your kit.

Step 1: Add the capacitor

Turn the generator amplitude down first.
Place the 100 µF capacitor between the + rail and the − rail, + leg on the + rail. Check the stripe.
Turn the amplitude back up and look at the MATH trace.

Measurement	Without capacitor	With 100 µF
Shape of the MATH trace (describe in a few words)
Highest point of the trace
Lowest point of the trace
U DC on the multimeter

Use CURSOR on the MATH trace to measure the top and the bottom of the remaining ripple accurately.

Quantity	Value
Ripple ΔU measured with the cursors

Step 2: Check it against the formula

The theory says:

\Delta U = \frac{I_{\text{load}} \times t}{C} \qquad \text{with } t = \frac{1}{2f} \text{ for a bridge}

Calculate the load current from your own measurement ( $I = U_{DC} / 1000$ ), take $f = 100$ Hz, and compare.

Step 3: A smaller capacitor

Replace the 100 µF with the 1 µF from your kit and look again.

Measurement	With 1 µF
Shape of the trace
Ripple ΔU
U DC on the multimeter

Name two changes to this circuit that would make the ripple smaller, and one change that would make it worse.

Put the 100 µF back for the next step.

2.9 An LED on the output of your power supply

You have built a small DC supply. Now use it.

Bridge rectifier with a smoothing capacitor and an LED as load

Step 1: Calculate the resistor again

Take the DC voltage you measured in step 2.8 with the 100 µF capacitor as your supply voltage, and calculate the series resistor for a red LED at 10 mA. Pick the nearest value above it from your kit.

Step 2: Build and measure

Replace the 1 kΩ load with your resistor and the LED in series, the LED's long leg towards the + rail.

Measurement	Predicted	Measured
U over the resistor
U over the LED
I through the LED
U DC across the whole load

Take a picture of the LED burning on your own rectifier.

Step 3: Slow it all down

Now make the rectifier visible with your own eyes instead of the scope.

Take the capacitor out of the circuit.
Set the generator to 2 Hz.
Watch the LED and count.

Situation	Flashes per second you count	Describe what you see
Bridge, no capacitor, 2 Hz
Half wave (pull out D2 and D3), no capacitor, 2 Hz
Bridge with 100 µF back in, 2 Hz

The bridge flashes twice as often as the half wave circuit at the same generator frequency. Explain why in one sentence.

Step 4: Find your own flicker limit

Put the bridge back together (all four diodes, no capacitor) and slowly raise the generator frequency until you can no longer see the LED flicker.

Quantity	Value
Frequency where the flicker disappears for you
Frequency where your neighbour stops seeing it

The mains runs at 50 Hz and a lamp on a bridge rectifier flickers at 100 Hz. Does that match what you just measured?

2.10 Extra: the RGB LED

Steps 2.10 to 2.12 are optional and are not graded.

The RGB LED in your kit is three LEDs in one package with one shared leg. Because the three colours have different forward voltages, they need three different series resistors for the same brightness.

Find out whether yours is a common cathode or a common anode type, using the diode mode of your multimeter.
Measure the forward voltage of each of the three colours.
Calculate a series resistor for each colour, for 10 mA on 5 V.
Build it and check the current in each branch.

Colour	U forward measured	R calculated	R used	I measured
Red
Green
Blue

Which colour needed the smallest resistor, and does that match the theory?

2.11 Extra: ripple against load current

Go back to the bridge with the 100 µF capacitor and try four different load resistors. Each time, measure the DC voltage and the ripple with the cursors.

R load	U DC measured	I load (calculated)	Ripple measured	Ripple from the formula
10 kΩ
4,7 kΩ
1 kΩ
220 Ω

Draw a small graph of the ripple against the load current. Does the measurement follow the formula?

2.12 Extra: other waveforms

Keep the bridge and the 1 kΩ load, remove the capacitor, and switch the generator between sine, square and triangle at 100 Hz.

Waveform in	Sketch or describe the MATH trace	U DC on the multimeter
Sine
Square
Triangle

Which waveform gives the highest DC voltage for the same peak value, and can you explain why?

3. Reflection

Write a short reflection of approximately half a page (font: Arial 9.5). Answer the following questions in your own words.

What did you learn?

Which concepts from the theory lesson did you recognise in the lab? Give a concrete example from your own measurements.

What was difficult?

Which step gave you the most trouble? How did you solve it?

The ground rule

Explain in your own words, as if to a fellow student who missed this lab, why a scope ground clip may not be put anywhere you like, and how you measured the load voltage instead.

Connection to IoT

Open the charger of your phone or your laptop and look at the label: it takes 230 V AC and gives a few volts DC. Name the parts of the circuit you built today that must be inside it, and name one thing that is in there which you have not built yet.